Packaged semiconductor devices generally comprise an IC die that is mounted on the die pad of a lead frame using a die attach adhesive. The die attach adhesive provides mechanical attachment, and generally also provides an electrical and/or thermal pathway to the die pad.
The die attach adhesive is generally a polyimide or epoxy based adhesive. Silver is often added in particle flake form as a filler to raise both the electrical and thermal conductivity of the material. The amount of silver is generally limited to about 80% by volume, because a silver content above this limit is well known to result in a loss of strength of the die attach material, and as a result an increase in cracking.
One reliability test for packaged ICs is Moisture Sensitivity Level (MSL) testing, which is used to establish floor life exposure and reflow condition limits for ICs. Although improvements in IC mold compounds and die attach materials have enabled improved MSL classifications at higher temperatures, stress-induced delamination and cracking of the die attach adhesive, which is typically in the thermal path under the IC die, is generally the leading package related failure for MSL testing. Delamination along surface breaking features is also known to be a reliability problem. Being unrepairable, delamination and cracking can lower package yield, and can also lead to field (e.g. reliability) failures.
One self-healing polymer composition is known for use as a die attach adhesive, such as for mounting IC die on a printed circuit board (PCB). In this known arrangement, the self-healing monomer and catalyst microspheres are disposed in a polymer matrix, along with optional metal particles. Although such self-healing die attach adhesives may reduce delamination of the die attach adhesive somewhat, such die attach adhesives have the same silver (or other metal) content restriction described above (<80 vol. %) that is required to maintain the required strength of the die attach material, which thus limits the obtainable electrical and thermal conductivity of the die attach adhesive.